As is well known, a circuit breaker is an automatically operated electro-mechanical device designed to protect a conductor from damage caused by an overload or a short circuit. A circuit breaker may be tripped by an overload or short circuit, which causes an interruption of power to the load. A circuit breaker can be reset (either manually or automatically) to resume current flow to the load. One application of circuit breakers is to protect motors as part of a motor control center (“MCC”). A typical MCC includes a temperature triggered overload relay, a contactor and a motor circuit protector (“MCP”). The MCP is a specialized circuit breaker that provides instantaneous protection against instantaneous short-circuit events. These motor circuit protector devices must meet National Electric Code (“NEC”) requirements when installed as part of a UL-listed MCC to provide instantaneous short-circuit protection.
Mechanical circuit breakers energize an electro-magnetic device such as a solenoid to trip instantaneously in response to a rapid surge in current such as a short circuit. Existing MCPs protect only a limited range of motors, but should avoid tripping in response to in-rush motor currents that occur during motor start-up while tripping on a range of fault currents including instantaneous short-circuit currents. In order to provide protection for a full range of motors with different current ratings, different MCP circuit breakers that match the operating parameters of the particular motor must be designed for each current rating. Each MCP circuit breaker is designed with specific trip point settings for a given current rating. Thus, many circuit breaker models must be offered to cover a full range of currents.
Circuit breakers may have current transformers that serve a dual function of measuring fault currents and supplying stored or instant energy to trip the breaker. The range of the circuit breaker is limited by the linear region of operation of the current transformer.
What is needed, therefore, is to provide a calibration process to use the saturation region of current transformers to increase the operating range of a circuit breaker. There is also a need for a calibration process that may be adjusted via programming without altering the basic test process.